My long-term goal is to understand the role and mechanisms of microRNAs in regulating tumor metastasis, and to develop new candidate therapies for malignant diseases. It has become increasingly evident that cancer pathogenesis can Involve a superfamily of small non-coding RNAs named microRNAs. While the oncogenic or tumor-suppressing functions of a number of microRNAs have been characterized, the role played by microRNAs in mediating metastasis was addressed only recently by work from myself and several other groups. In my initial screening, I identified three microRNAs that are most significantly upregulated in human breast cancer cell lines: mlR-155, miR-9, and miR-10b. Subsequent funcitonal experiments demonstrated that overexpression of miR-10b induced tumor invasion and distant metastasis in two orthotopic models of breast cancer. In the K99 phase of this award, I discovered that therapeutic silencing of miR-10b with 'antagomirs'suppressed metastasis in a mouse mammary tumor model. In addition, I identified miR-9 as an E-cadherin-suppressing and metastasis-promoting microRNA. In the R00 phase, I will extend these previous-studies by using molecular, genetic, pharmacological, and genomic approaches. Specifically, I will pursue the antagomlr-10b study by using multiple models and testing combination therapies In mice;I will establish a genetically engineered mouse model to determine the role and mechanisms of miR-10b In normal development and In metastatic progression of spontaneous breast cancer;and I will explore the cell non-autonomous effects of miR-10b In metastasis formation. In parallel, I will perform pre-clinical studies with the miR-9 antagomir;and I will also investigate E-cadherin-independent functions of miR-9 in tumor cells. Taken together, these studies will enable more precise evaluation of the role and mechanisms of these microRNAs In malignant progression, and will allow me to launch my independent research program and obtain preliminary data for R01 application.